It is known (U.S. Pat. Nos. 4,431,009 and 4,335,729) that the parasitic oscillations can be attenuated by providing dampers in the vicinity of the pressure sensor in parallel with the transmission line. The dampers absorb part of the high frequency components of the pressure signal thereby moderating the amplitude of the reflected oscillations.
The damper, according to U.S. Pat. No. 4,431,009, is embodied in an adjustable needle valve which represents a flow resistance that can be varied. The damper is arranged as a separate aggregate between the pressure measuring transformer and a three-way valve. This valve links the transmission line between the patient and the pressure sensor to an infusion means because in the majority of cases when blood pressure is continuously measured the transmission line is rinsed with an infusion solution. Finally, a check valve is also located in the transmission line so it can cut off the pressure sensor from the measuring system. This is required to adjust the pressure sensor to atmospheric pressure.
The damper, according to U.S. Pat. No. 4,335,729is likewise designed as a needle valve and is connected to a branch-off from the transmission line. It includes a sealed compartment totally surrounded by rigid walls and containing an air cushion to dampen the oscillations. The flow cross section of the connection between the transmission line and the compartment is regulated by the adjustable needle valve.
These dampers serve their purpose, however they are rather expensive for disposable items. Moreover, the additional adjustable component makes both measuring systems complex because they need to be continually adjusted. Finally, their structural design require the dampers to be mounted only at a certain distance from the pressure transducer so that their effect is not optimal.
German patent DE 24 05 584 responds to the problem of reflections in a system for the pulse-wise ejection of droplets by suppressing the reflected pressure wave with acoustic impedance matching through an elastic conduit.
German patent DE 29 41 118 shown a liquid spring damper comprising of two pot-like compartments which are supported "floatingly" with respect to each other by a shear spring. The two compartments communicate through a throttle. An elastic bellow is arranged inside the inner compartment and, as it is pressurized by adjustable gas pressure, it blocks the throttle at an appropriate gas pressure. The throttle does not open until the pressure n the main compartment exceeds that in the bellows. In this manner, a damper is provided which has a nonlinear characteristic and is adjustable by the pressure inside the bellows.